RAMAN INTENSITIES INDUCED BY ELECTROSTATIC INTERMOLECULAR INTERACTIONAND RELATED NONLINEAR-OPTICAL PROPERTIES OF A CONJUGATED PI-ELECTRON SYSTEM - A THEORETICAL-STUDY

Raman intensities of a charged conjugated,pi-electron system induced b y electrostatic intermolecular interaction are studied theoretically. By using a simple Hamiltonian based on a two-state model, in which the response of the system to an electric field is taken into account, th e formulas for the polarizability derivative and related quantities ar e derived. These formulas are applied to the case of a pentamethine st reptocyanine dye, which has a symmetric conjugated chain consisting of four CC bonds with one NC bond on each end. It is shown that a reason able magnitude of electrostatic interaction with a counterion induces Raman intensities on the order of 10(2) Angstrom(4) amu(-1) for the mo des with large contributions from the vibration along the bond-alterna tion coordinate of the chain, explaining the appearance of the 1574- a nd 1207-cm(-1) bonds in the Raman spectrum measured in solution. A sli ght deformation of the conjugated chain along the bond-alternation coo rdinate induced by electrostatic interaction and the strong electron-v ibration interaction are responsible for these Raman intensities. The formulas derived in this study are then used for evaluating the electr onic and vibrational contributions to the first hyperpolarizability. I t is concluded that the vibrational contribution is on the same order of magnitude as (but smaller than) the electronic one, at least in the case of typical charged conjugated pi-electron systems.